The Harpoon is a flail weapon that can only be acquired from the Goblin Army event. It can be dropped from any of the goblins during the event. It is a ranged weapon that fires a harpoon on a chain. It does not require ammo and is similar to the Ball O' Hurt, with the exception that the harpoon is smaller than the ball, with a longer reach, thus is harder to aim as it has a distinctive arc and will retract when it has reached its maximum length, or when it collides with a surface or enemy.
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Terraria Harpoon

The Harpoon does 25 ranged damage and attacks with different speeds depending on the distance. It can fire as fast as a Minishark provided the player is firing from a 1 block distance.

With flails, a good strategy when facing a larger group of enemies is to get to an elevated position (or jump) and shoot at a target from the distance, as the projectile will then hit said enemy and, while coming back, damage everything it touches.

The Harpoon acts as a mass on a chain, and is affected by gravity. As such, it is useful for firing over walls to hit enemies on the other side. The weapon will not latch into the target and pull it back towards the player, but will simply hit them and return back.

Harpoons can be shot straight through Wood Platforms, making it easy to safely attack from above.

If the player is facing an enemy up close while the flail of the Harpoon is away, it will be hit with normal Harpoon knockback.

This shows that you must aim above your target to hit it from range, as the harpoon is affected by gravity.

Trivia

The harpoon is the only flail that deals ranged damage instead of melee damage.

Notes

If used with good aim, accompanied by its fast return, the Harpoon can be a good weapon against flying enemies like Vultures,Bats, or even the Eye of Cthulu.

In mobile versions the harpoon now has a 1 second cool down which make this weapon less efficient than it used to be.